This invention relates to nuclear reactors and in particular to an apparatus for holding down fuel assemblies within the reactor core.
In pressurized water reactors the coolant flow rate and fuel assembly flow resistance are such that the hydraulic uplift force is of sufficient magnitude to cause the assemblies to jitter and even lift off the core support structure. Various approaches have been used to eliminate this detrimental movement.
One suggested solution involves the use of a lock down device which attaches the lower end of the fuel assemblies to the core support structure. While this device will function properly, it does introduce mechanical complexity since the device must not only lock and unlock remotely, but it must release reliably after a year of operation in the reactor environment.
Another approach has been to use springs located above each fuel assembly which bear against an upper alignment plate, thereby urging the fuel assemblies down. As reactors have been designed with increasingly large hydraulic uplift forces the spring force has become very large. The springs themselves have become so large that an excessively large plenum is required between the alignment plate and the upper end fitting of the fuel assembly. The springs at this location produce an undesirable flow pattern and an excessively high pressure drop through the plenum. They are also potentially subject to flow induced vibration since the total reactor coolant passes over these springs.